1. Field of the Invention
The present invention relates to cordierite ceramic honeycomb, particularly cordierite ceramic honeycomb having a high thermal shock resistance, a high thermal resistance and an excellent catalyst adhering ability and a high catalytic property, and a method for producing the ceramic honeycomb.
The ceramic honeycomb means a thin-walled multicellular, that is, honeycombed structure composed of a ceramic material and the cross-section of the cell may be any geometrical pattern, such as, hexagonal, square, rectangular, triangular, and circular shape.
A mean pore size which is a property relating to the pores in the thin wall of the ceramic honeycomb, is defined as a pore diameter corresponding to 50% of the total pore volume in the pore size distribution according to mercury pressure porosimeter tests.
2. Description of the Prior Art
The ceramic honeycomb has been used as a catalyst substrate to be used for an apparatus for purifying hydrocarbons, carbon monoxide and nitrogen oxides in an automotive exhaust gas. The ceramic honeycomb catalyst substrate to be used for the apparatus for purifying the automotive exhaust gas requires several important properties. One of these properties is thermal shock resistance, which means that no crack or breakage is caused by thermal stress caused by a large temperature difference which occurs in the ceramic honeycomb subjected to the temperature change due to rapid heat generation in a catalytic oxidation reaction of unburned hydrocarbon and carbon monoxide in the exhaust gas. When a honeycomb is durable to rapid heating and quenching, through a temperature difference exceeding about 500.degree. C., which is an indication of this thermal shock resistance, it has been known that there is no practical problem. The smaller the thermal expansion coefficient, the higher the temperature to which a material is durable to rapid heating and quenching, so that it has been known that the thermal expansion coefficient has the highest influence upon the thermal shock resistance among the properties of the ceramic honeycomb and the ceramic honeycomb having a low thermal expansion coefficient has been demanded. The other property required in the ceramic honeycomb catalyst substrate is thermal resistance, which is high temperature stability showing resistance against fusing damage upon misfire of an engine. When a raw gas caused by misfire of an engine is introduced into the honeycomb catalyst having a normal operating temperature of about 500.degree. C. and a rapid oxidation exothermic reaction occurs in the catalyst, if the thermal resistance is low, the honeycomb is melted and the passage resistance of the exhaust gas becomes larger and the engine is subjected to a large load. However, automobiles provided with the catalytic purifying apparatus have been generally provided with a safety device, such as a secondary air controlling mechanism for controlling the misfire upon idling, driving under a high load, driving at a high speed and driving on a downward slope, so that unless the ceramic honeycomb to be used as the catalyst substrate is softened and shrunk at 1,450.degree. C., there is no practical problem. Another property required for the ceramic honeycomb catalyst substrate is an adhering ability, which is the adhering and supporting ability upon coating an active material for catalyst and a catalytic component on the honeycomb catalyst substrate and an adhering and holding ability of the active material for catalyst and the catalytic component on the catalyst substrate by which the coatings are not exfoliated upon driving.
Heretofore, as the material for the ceramic honeycomb structure, use has been made of cordierite, mullite, alumina, zircon, lithia and so on, carbides and nitrides. Among them, cordierite, mullite, alumina and zircon have been used in view of the thermal resistance and antioxidation as the material for the ceramic honeycomb catalyst substrate for purifying engine exhaust gas.
The ceramic honeycomb made of mullite, alumina, zircon or a mixture thereof is better in the thermal resistance than the cordierite honeycomb and satisifies practically in the high temperature stability at about 1,450.degree. C. but the thermal expansion coefficient is 3-5 times larger than that of the cordierite honeycomb, so that the thermal shock resistance is poor and when the temperature change due to a rapid catalytic oxidation exothermic reaction of unburned hydrocarbon and carbon monoxide in the exhaust gas is applied and a large temperature difference is caused in the honeycomb, cracks and breakage are caused in the honeycomb due to the thermal stress.
On the other hand, the cordierite ceramic honeycomb shows a low thermal expansion coefficient as disclosed in Irwin M. Lachman et al, U.S. Pat. No. 3,885,977 issued May 27, 1975 and entitled "Anisotropic Cordierite Monolith", so that the cordierite ceramic honeycomb is excellent in the thermal shock resistance but is lower in the melting point than the mullite honeycomb and alumina honeycomb and is suddenly softened or melted at a temperature of higher than about 1,400.degree. C. and therefore when the temperature of the honeycomb is raised to about 1,450.degree. C. by misfire of a engine, the honeycomb shape cannot be maintained.
The ceramic honeycombs produced by extrusion as disclosed, for example, in John Jones Benbow et al, U.S. Pat. No. 3,824,196 issued July 16, 1974 and entitled "Catalyst Support", Rodney D. Bagley, U.S. Pat. No. 3,790,654 issued Feb. 5, 1974 and entitled "Extrusion Method for Forming Thin-walled Honeycomb Structures" and U.S. Pat. No. 3,905,743 issued Sept. 16, 1975 and entitled "Extrusion Apparatus for Forming Thin-walled Honeycomb Structures", which is divided from Bagley, are dense in the texture by extrusion forming under a high pressure and the total pore volume becomes small and at the same time the mean pore size in the thin wall, particularly the surface of the thin wall becomes small, so that the adhering ability of the active material for catalyst and the catalytic component on the catalyst substrate becomes poor and the active material and the catalytic component are liable to be exfoliated during use. Moreover, in the production of the honeycomb through extrusion forming, when the plasticized batch is formed into a honeycomb and the formed honeycomb is dried and fired to form the ceramic honeycomb, if finely divided starting material is used as the starting material for the honeycomb or starting material containing crystal water or a salt, such as carbonate, sulfate, nitrate and the like is used, shrinkage is large in the drying and ring steps, so that cracks are liable to be caused in the drying and firing steps and the yield is poor.
The ceramic honeycomb catalyst substrate to be used in the apparatus for purifying exhaust gas of automobiles is very severe in the using condition and the ceramic honeycomb having a high thermal shock resistance, a high thermal resistance and an excellent adhering ability of the active material for catalyst and the catalytic component on the catalyst substrate has been strongly demanded.